Siliconized alpha-delta ferrous alloy



United States Patent Oiice 3,438,802 Patented Apr. 15, 1969 3,438,802SILICONIZED ALPHA-DELTA FERROUS ALLOY Jerome J. Kanter, Palos Park,Ill., assigner to Crane C0., Chicago, Ill., a corporation of IllinoisFiled May 25, 1962, Ser. No. 197,702 Int. Cl. B44d 1/09; C23c 13/02 U.S.Cl. 117-106 6 Claims This invention relates to the impregnation ofmetals with silicon, and especially to the formation of an adherentcoating, or case, containing silicon upon ferrous metal articles.

It is well-known that silicon-iron alloys yare not only resistant tomechanical wear and abrasion, but are also corrosion resistant whichmakes then highly desirable for certain industrial uses. But such alloysare also brittle, of relatively low strength and toughness, anddifficult to machine. For this reason, although their exceptionallydesirable properties might have resulted in extensive industrial use,their concomitant undesirable properties has limited their use.

Attempts have been made by workers in the eld to produce articles havingsurface coatings, or cases, of high silicon-iron alloys adhering tostrong tough cores of iron or steel. The desired articles are firstfabricated in part or substantially to their desired final form fromiron or steel of the desired strength, toughness and other mechanicalproperties. Then the surfaces of the articles, or some selected portionsthereof, are siliconized to cause impregnation, penetration, ordiffusion of silicon into the metal of a sufficient depth to provide aprotective skin, layer, or case. This process is illustrated by thefollowing United States patents: Ihrig Patent Nos. 2,163,753; 2,142,941;2,157,902; and 2,109,485; Henderson Patent No. 2,501,051; Eckman PatentNo. 2,897,093.

In spite of the -attractive potentials of the siliconizing processesillustrated by these patents, commercial utilization has been limited.Aside from other defects, the siliconized products have beencharacterized generally by poor adherence between the case containingsilicon and the underlying core. As a result, siliconized articles havea pronounced tendency for the case to spall off, or for cleavage betweenthe two layers. Spalling, or cleavage, is especially apparent uponsubjecting the articles to wide changes in temperature, mechanicalshock, and the wide variety of usages encountered in actual serviceconditions.

Commercial ulitization has been limited also to a restricted class offerrous metals. Workers in the eld have heretofore believed that onlycarbon steels were commercially satisfactory, and generally regardedalloy steels unsatisfactory. The reason for this attitude has been dueapparently to the fact that some processes in which free chlorine waspresent, the chlorine reacted with chromium in the steel alloys to forman undesirable refractory coating of chromium chloride.

A general object of the invention, therefore, is to avoid thedifficulties and objections characterizing prior siliconizing processes,and to produce articles containing a silicon case superior to thoseheretofore obtainable. A more specific object of the invention, and oneof importance, is to produce ferrous metal yarticles having a firmlyadherent case, or coating, containing silicon, which is less susceptibleto spalling or cleavage between the case and the underlying core. Afurther specific object of the invention is to produce ferrous metalarticles having a high-silicon case on bases, or cores, of iron or steel`alloys of compositions generally regarded as impractical. It is a stillfurther specific object of the invention to reduce the scrap loss insiliconizing processes, It is a yet further object of the invention toproduce a case containing silicon of high density. It is a still yetfurther object of the invention to produce siliconized productscharacterized by fewer microiissures. It is a still yet 'further objectof the invention to provide siliconized -articles having a highercorrosion resistance than obtainable herebefore. Another object of theinvention is to provide a process for siliconizing steel alloys. Furtherobjects and advantages of the invention will be apparent. from a studyof the description and appended claims.

Briefly stated, the present invention is directed to siliconizingferrous base articles which have a stable body centered cubic crystalstructure, for example, alpha-delta alloys. More particularly theinvention is directed to siliconizing ferrous base articles that arefree from phase transformations up through the siliconizingtemperatures, usually in the range from 1300 F. to about 2000 F.

The accompanying drawing illustrates diagrammatically in cross-section aproduct prepared in accordance with the present invention in which aferrous article having a body centered cubic crystal structure forexample, alpha-delta alloy, is siliconized to form an adherent case orcoating.

The siliconizing process employed may be one, or more, of the processesset forth in the Ihrig, Henderson, or Eckrnan patents set forthhereinabove, `although the Eckman process is preferred. These processesare carried out by heating the article to be treated to elevatedtemperatures, usually in the range from 1300" F, to about 2000 F., moreparticularly from about 1500 F., to `about 1900 F., generally above 1650F., and, in most instances, at around 1850 F. At elevated temperaturesthe articles are contacted with siliconizing reagents. The siliconizingreagents may be solids, gases, or combinations of the foregoing,including the reagents given in the above patents.

According to the present understanding of the chemical mechanismsinvolved, each of the processes forms silicon tetrachloride which reactswith the surface of the metal to form silicon and luy-product chlorinegas. The elemental silicon thus formed inoculates the outer layer, orportion, of the treated ferrous article.

According to one procedure, the articles receiving the siliconizingtreatment are placed in a horizontal drum type retort. The retort isconnected to a source of substantially dry inert gas, such as nitrogen,neon, or argon, which is introduced to displace the air. After thepreliminary flushing the ow of nitrogen, for instance, is reduced so asto be sufficient to make up for losses that may occur due to leakage.Then the retort is rotated and heat is applied, for example byelectrical heating elements on the sides, to raise the temperature toproper siliconizing levels. The temperatures are not critical, butgenerally best results are obtained at approximately 1850 F.

After the siliconizing temperature is attained, the flow of the inertgas is shut off and silicon tetrachloride gas is introduced into theretort. This gas is generated by applying heat to a receptaclecontaining liquid silicon tetrachloride. Alternatively, the silicontetrachloride may be introduced by a carrier gas which is bubbledthrough the silicon tetrachloride liquid and then conducted into theretort. After the retort has filled with the siliconizing reagent, withor without the presence of the carrier, the flow of the reagent isreduced to a slow, continuous in- -gress for the siliconizing process.

In the preferred process, as more particularly described in the UnitedStates patent to Eckman given above, silicon carbide is added to theretort, so that the reaction between the siliconized article and thesilicon tetrachloride takes place in the presence of silicon carbide.The silicon carbide controls, or reduces, the reaction with the iron. Inan alternative process, as more particularly described in the UnitedStates patent to Henderson given above, hydrogen is introduced into theretort. The foregoing and other agents, or controls, may be employed asdeemed expedient.

Ordinarily, the exposure time of the treated articles to thesiliconizing atmosphere is from about 0.5 to about ve hours. Usuallysatisfactory results are obtained within one to three hours.

By controlling the time of exposure to the siliconizing atmosphere, andthe temperature, the depth of silicon impregnation and concentration ofsilicon in the case may be controlled according to need.

After receiving the siliconizing treatment, the articles may be allowedto cool in the retort to room temperature, or may be removed to separatecontainers to lower temperatures for handling.

The present invention is based on the discovery that ferrous basearticles having a stable body centered cubic lattice can be siliconizedto form a silicon-containing coating, or case, with many uniqueadvantages, including those set forth hereinabove. Preferably, theferrous articles must have a stable body centered cubic microstructurewhich is not `transformable into allotropes. Most desirably, the ferrousarticles will not have allotropic transformations in the temperaturerange from ambient temperatures to siliconizing temperatures, the latterOf which may be as high as about 2000" F.

The stable alpha-delta iron alloys, which maintain a body centered cubiclattice structure throughout the solid phase, are well known, and onlyexemplary alloys are set forth below to illustrate ferrous base alloyswhich may be siliconized to produce the unique advantages of theinvention. In general, the alloys may be utilized which containconstituents of the type, and in the amount, which close the gamma loopof ferrous articles, for example, silicon, tungsten, aluminum, vanadium,molybdenum, chromium, titanium, and the like.

Only small concentrations of silicon, for example, at least about twopercent by weight are needed to produce a stable alpha-delta alloypossessing a body centered cubic microstructure throughout the solidphase. A siliconized case formed on this alloy reduces the tendency tospall.

Titanium exerts a strong influence in closing the gamma loop, and merelyat least about 0.75% by weight is required to produce a stablealpha-delta alloy.

Tungsten in amounts of at least about six percent by weight is needed toclose the ygamma loop. A siliconized case formed on this alloy hasimproved adherence of the case to the core.

Only a small concentration of aluminum of at least about one percent byweight is required to stabilize and form the alpha-delta alloy. Asiliconized coating, or case, formed on an iron alloy of this typereduce the tendency to spall, and is characterized by exceedingly highcorrosion resistance.

Only at least about 1.5% by weight of Vanadium is required to close thegamma loop. This alpha-delta alloy, when siliconized, has an improvedadherence between the case and the underlying core.

Only at least about three percent by weight of molybdenum is required toclose the gamma loop of iron. Molybdenum steel alloys reduce thecleavage between the siliconized case and the core.

Only at least about two percent by weight of niobium is needed to form astable alpha-delta alloy.

Chromium closes the gamma loop of iron with at least about thirteenpercent by weight concentration. Steel alloys containing chromium abovethese amounts produce improved siliconized cases.

It is to be understood that the alpha-delta alloys are not equallydesirable, or useful. The alloying constituents themselves are more, orless, desirable for various intended objects and uses. By way ofexample, alloying constituents may be classified into two -groups as totheir reactivity with chloride; (A) those which are more reactive withchlorine than iron, for instance, cerium, yttrium, magnesium, aluminum,manganese, titanium, and chromium; and (B) those which are less reactivewith chlorine than iron, for instance, carbon, tungsten, molybdenum,nickel, cobalt, and copper. The alloying constituents of group (A) abovetend to form spongy porous cases as their concentration increases n theprocess of the above-identified Eckman patent. Chromium, for instance,in concentrations above 5% by weight results in a thick spongy casewhich increases in thickness and sponginess in relation to the chromiumcontent. The alloying constituents of group (B) above tend to formdenser cases than those of group (A) in high concentrations, in theprocess of the above-identified Eckman patent.

In some instances it may be desirable to employ several alloyingconstituents to close the gamma loop. For instance, at least aboutthirteen percent by weight of chromium is needed to form an alpha-deltaalloy. At these concentrations the case tends to be spongy, even thoughthe tendency to spall off is reduced. Nevertheless, if a chromiumcontaining alpha-delta alloy is desired, the chromium may be maintainedin concentrations below about five percent by weight to avoid a spongycase, and other alloying elements may be used to close the gamma loop,such as titanium, aluminum, silicon, or the like. Various combinationsof alloying elements which will attain a stable alpha-delta alloy, withother desirable properties for the intended uses, will be apparent tothose skilled in the field.

While not intended to be limited thereby, the theoretical considerationswhich are believed to produce the improved siliconized case, are asfollows. The siliconized case, or coating, contains Fe3Si, which has acrystal structure of ferrite, body centered cubic with a double latticeparameter. Siliconizing has ordinarily been produced upon articles thatare austenitic at siliconizing temperatures. Despite the transformationof austenite to ferrite in the base materials, with the accompanyingvolume increase on the order of 5%, the siliconized case containingFe3Si remains adherent in many alloys. This is probably due to plasticflow or creep, which diminishes the shear force and maintains theadherency between the siliconized case and the core.

In the event, however, iron or steel is alloyed to the extent thatsuppressed transformation occurs at a black heat, below the creep rangeof steel, the shearing modulus and the shearing stress is substantiallyincreased and probably accounts for spalling in many instances when thearticle is subjected to wide changes in temperature, or mechanicalshock.

In the present invention, there are no phase transformations, and thus,no Volume changes in the underlying core that produce shear forcesbetween the case and the core, which tend to spall the case.Furthermore, the underlying core has the same type of microstructure, abody centered cubic lattice, as Fe3Si produced in the case.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims, or the equivalent ofsuch be employed.

I claim:

1. As a new article of manufacture, a ferrous base metal alpha-deltaalloy having a stable body centered cubic crystal structure insufficient amounts in its solid solution phase to suppress allotropictransformations in the temperature range up to about 2,000" F., saidferrous base metal alpha-delta alloy provided with an adherentsiliconized case.

5 6 2. The new article of claim 1 in which said alpha-delta ReferencesCited alloy contains at least about 2% by weight silicon.

3. The new article of claim 1 in which said alpha-delta UNITED STATESPATENTS alloy contains at least about 0.75% by weight of titanium.2,501,051 3/1950 Henderson et al 4. The new article of claim 1 in whichsaid alpha-delta 5 FOREIGN PATENTS alloy contains at least about 1% byweight aluminum.

5. The new article of claim 1 in which said alpha-delta 543495 7/1957Canada' alloy contains at least about 3% by weight basis of molyb- RALPHS KENDALL Pmary Examiner denum.

6. The new article of claim 1 in which said alpha-delta lo U-S- C1- XR.

alloy contains at least about 3% by weight niobium. 117-1351 U.S.DEPARTMENT 0F COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITEDSTATES PATENT OFFICE CERTIFICATE 0F CORRECTIQN Patent No. 3,438,802April l5 196 Jerome J. Kanter It is certified that error appears in theabove dentfed patent and that said Letters Patent are hereby correctedas shown below:

Column 4, line 5, "chloride" should read chlorine Signed and sealed this7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. AS A NEW ARTICLE OF MANUFACTURE, A FERROUS BASE METAL ALPHA-DELTAALLOY HAVING A STABLE BODY CENTERED CUBIC CRYSTAL SSTRUCTURE INSUFFICIENT AMOUNTS IN ITS SOLID SOLUTION PHASE TO SUPPRESS ALLOTROPICTRANSFORMATIONS IN THE TEMPERATURE RANGE UP TO ABOUT 2,000*F., SAIDFERROUS BASE METAL ALPHA-DELTA ALLOY PROVIDED WITH AN ADHERENTSILICONIZED CASE.